In a conventional case of emitting third and higher-order harmonics, a configuration of using not only a first wavelength conversion crystal for emitting a second harmonic but also a second wavelength conversion crystal for emitting a third or fourth harmonic has been generally employed.
However, such a configuration of using a plurality of wavelength conversion crystals requires not only a plurality of wavelength conversion crystals but also adjustment means for adjusting an angle and a temperature of each wavelength conversion crystal, and also other adjustment means for adjusting an incidence angle and an incident beam diameter of light to be wavelength-converted and to be incident on each wavelength conversion crystal. These requirements may raise costs required for the wavelength conversion crystals and the wavelength conversion laser device.
In addition, since it is required to adjust angles and temperatures of the plurality of wavelength conversion crystals and an incidence angle and a beam diameter of light to be wavelength-converted, the configuration has problems of not only requiring much time and effort for the adjusting works but also causing an output, a beam intensity distribution, and a beam diameter of a higher-order harmonic to easily fluctuate due to variation of an appropriate value of each of elements to be adjusted.
To address these problems, a technique of integrating the plurality of wavelength conversion crystals by using such a method as optical contact has been proposed (for example, see Patent Document 1). Also, another technique of emitting a third harmonic from a single wavelength conversion crystal by using a specific crystalline material has been proposed (for example, see Patent Document 2).